Polyurethane foams, having ASTM E-84 flame spread of 75 or less, and in particular, 25 or less, have been prepared from special polyols, such as phosphorous-based, halogen-containing polyether and/or polyester polyols. The polymerization reaction has been carried out in bulk and in solution to provide essentially cross-linked brittle and friable foam products. The foam products are produced by combining the special polyols, an inert blowing agent, such as a low-boiling-point liquid like fluorocarbons, and one or more catalysts like a tertiary amine, and optionally a cell-control agent, and heating to effect the polyurethane reaction. The foams so produced to date have been poorly accepted, and have been characterized by high brittleness and high friability. Brittleness refers to the internal friability of the foam structure which remains essentially unchanged with time; that is, it is structural and molecular in nature, while friability refers to the state of the surface of the polyurethane foam; that is, the powderability of the surface when subject to pressure, which friability changes with time.
Attempts to reduce the friability of polyurethanes are made by modification of the polyurethane principally through the introduction of longer chemical linkages. This can be effected by the use of additional polyols which can be characterized as having low functionality (less than 3-4) and/or low hydroxyl content (hydroxyl number less than 400). This type of modification typically reduces overall physical properties and, more importantly, has an adverse effect on the flammability characteristics, such as smoke-generation properties. The best, present day, commercial products utilize nitrogen-based modifiers (so called amine-polyols) having typical hydroxyl number of 300-500 and functionality of 3-4.
It is desirable to prepare polyurethane foams which are capable of receiving ASTM E-84 flame-spread values of 25 or less and smoke-generation values with 4-inch foam samples of 400 or less. ASTM E-84 is commonly referred to as the "Steiner tunnel" or the tunnel test. Tunnel test values are made as comparisons to standardized values of 0 (zero) for asbestos cementboard and 100 for a particular grade of oak wood. The friability of polyurethane foams has severely limited the potential commercial success of this class of polyurethane foam products, because of the difficulty of processing the foam. A standard polyurethane foam polyol of hydroxyl number greater than about 500 typically produces a friable polyurethane foam with poor adhesion qualities, especially to unheated metal substrates. There are several techniques available to reduce the friability of such foam products, which techniques include the aforementioned reduction in hydroxyl content, increased catalyst concentration and the use of heated substrates. Plasticizers which are incorporated to reduce friability, such as organic plasticizers like phthalates; for example, dioctyl phthalate, often and typically significantly increase the flammability characteristics of the foam products to an unacceptable level of flame-spread values of more than 25, and high smoke-generation values. In addition such plasticizers degrade the physical properties of the foam, because of the high level use requirement necessary to produce a nonfriable foam product. Although the ASTM E-84 test procedure has not been shown to relate to actual fire conditions, it is used as a comparative value for polyurethane foam panels (and other building products) tested under similar fire conditions, and is one of the guides in judging the commercial acceptability of polyurethane foam under flame or fire conditions.
It is, therefore, most desirable to provide polyurethane foams which have low friability while retaining the other desirable properties of such foams, particularly with regard to low flame-spread values of less than 25 and low smoke generation, and methods of preparing such foams which provide for improved and rapid process conditions and cure of the polyurethane foam.
In the past, various modifiers have been employed in connection with rapid-setting, solid, dense, noncellular, polyurethane plastic compositions in order to modify the copies of such compositions, such as set forth, for example, in U.S. Pat. Nos. 3,883,466; 3,878,156; 3,878,157; and 3,886,102, and in different-type polyurethane foams for different purposes in U.S. Pat. Nos. 3,751,392 and 3,074,894.
Accordingly, the present invention is directed to the field of polyurethane foams of both improved friability and acceptable ASTM flame-spread properties, as well as foams which maintain high physical properties typical of highly cross-linked, rigid foams.